Originally posted by moving_electron What is the correct way to calculate the effect of the 22pF cap across the 220k feedback resistor? Searching through some other threads it looks like the feedback was being filtered low pass at ~40kHz?

I was not having much luck finding the bandwidth calculation for the feedback resistor/capacitor combination. Your post helped me find the info.

Here are some thoughts about solving my problem:

1) Using a 220pF ceramic cap between Vin+ and Vin- is the preferred way to reduce or eliminate radio frequency problems before reducing the amp bandwidth. Bypassing on the power supply to eliminate incoming high frequency from that direction is necessary as well.

I used 100pf and it worked for me but I will probably change this to 220pF because a) it will lower the frequency point above which unwanted signals are attenuated and b) no one seems to report any negative effect of the 200pF. It makes me think it is pretty much always worth doing as standard practice. The amp may be near a radio tower at some point on its life.

All my work on the amp was with the case open so it is likely that things would have gotten better with the grounded case closed up. Although for me, past history and the close proximity to radio towers makes me think I still would have seen problems without the 100pF-220pF.

2) At some point in my trouble shooting I inadvertently put 220pf from the Vin- to ground. This can definitely cause instability as described in the link that digi01 posted above. The effective resistance to ground on the input goes low at high frequencies, lowering the amp stability point, causing the amp becomes unstable at those frequencies.

3) The link to the above TI paper caused me to look for other TI papers. The simplified calculation for the effect of a capacitor across the feedback resistor can be found in another TI paper http://focus.ti.com/lit/an/sloa058/sloa058.pdf on page 13 in section 3.1.1. The discussion in the paper is about single supply circuits but I believe the equation applies.

The corner frequency of the inverting amp which has a capacitor in parallel with the feedback resistor (essentially a low pass filter with gain) is:

Fo = 1/(2pi(feedback resistor)*(parallel capacitor)

For a 200k resistor in parallel with a 22pf cap this gives:
Fo = 1/(2*3.14159*(200,000)*2.2E-11 = 32.9 kHz

As pointed out early in this thread that is pretty low. Presumably there are exhaustive threads on this (slew rate / bandwidth).

There is a recommendation in a previous threads that for a 100k resistor on an inverted amp a 10-22pf cap can be used to limit bandwidth as a last resort to eliminate oscillation. These values would result in:

Fo (for 100k/22pF case) = 72.3k Hz
Fo (for 100k/10pF case) = 159k Hz

So to get similar bandwidth with a 220k feedback resistor one would use a 5pF-10pF capacitor.

This does give one an appreciation of how stray capacitance on a PC board or parts can effect bandwidth at high frequencies.

For clarity: Here is the equation with the proper closed parenthesis added

Fo = 1/(2pi(feedback resistor)*(parallel capacitor))

I am glad to say this amp is *very* quiet now, even though I hooked it up to a computer soundcard and also to a tv with game boxes hooked to it. These sources have given me hum problems before but things are very very quite with this amp.